1. Field of the Invention
The present invention relates to power conversion, and corresponding devices and systems, that senses current and adjusts a regulated current being delivered to a load. More particularly, certain embodiments of the present invention relate to power conversion within a light emitting diode (hereinafter, “LED”) system that senses relatively low current on a switch/sense node and relates this sensed current to the amount of regulated high current being delivered to an LED string(s).
2. Background of the Invention
The benefits and wide-range applicability of LEDs in today's lighting systems are now realized and recognized by those skilled in the art. For many years, halogen-based lamps were the primary light source implemented within lighting systems. Over the past years as LED technology has developed, the advantages of LEDs over halogen lamps have become increasingly apparent. When compared to halogen lamps, LEDs are relatively smaller, and have a longer operating life. Another important difference between halogen bulbs and LEDs is the significantly less amount of power required by LEDs to operate. For example, a halogen lamp may operate within a range of 20-50 Watts and an LED at about 5-15 Watts.
When LEDs are used for lighting applications, a cluster or an array of LEDs is used to achieve the requisite brightness and other desired lighting characteristics. Regardless of color, type, color, size or power, all LEDs work the best when driven with a constant current. LED manufacturers specify the characteristics (such as lumens, beam pattern, color) of their devices at a specified current value. One or more LED drivers are used to effectively control the electrical characteristics of the array of LEDs to suit the lighting. A LED driver is a self-contained power supply that has outputs matched to the electrical characteristics of the array of LEDs. Most LED drivers are designed to provide constant currents to operate the array of LEDs.
Many LED lamps are powered in the same way as other lighting applications, namely, starting with and using an alternating current (AC) power source. Depending on the geographic location or application, the AC source could range between 100V and 240V. The frequency of these AC sources ranges between 50 Hertz and 60 Hertz. To meet energy star requirements for LED lighting applications, the required power factor has to be greater than 0.9. This can be achieved by a passive or active power factor correction circuit.
In applications where the power levels are higher than 25 Watts, an active power factor correction circuit is typically used to provide a regulated high voltage DC bus. This regulated bus is used to power the LEDs by a power conversion circuit. This power conversion circuit may be an isolated topology or non-isolated topology.
Several LED lighting applications that operate within high voltage DC or AC ranges require that the current delivered to the LED be measured. In many applications, the LED is at a high voltage and sensing the LED current requires relatively expensive high-side current sense amplifiers or current sense transformers to measure the current flowing into the LEDs. This sensed information is subsequently sent to the control side of the driver so that the regulated current may be adjusted if appropriate. In applications where the LEDs are positioned within an isolated topology, optical couplers may be used to transfer the LED current information from the systems secondary side to the primary side.
This requirement in prior art systems to measure current on high current lines feeding into the LEDs and/or on lines isolated from the control side of the driver requires expensive sensing components within the system and possibly expensive optical couplers. What is needed is a system and method that eliminates high current sense components (e.g., high current sense amplifiers or transistors and optical couplers) within LED systems. This need is relevant in both non-isolated topologies as well as isolated topologies.